The invention relates to a frequency detector for use in a radio telephone.
During a telephone call placed in both the AMPS and TACS radio telephone systems, a base station transmits an analog sinusoidal control signal, a so-called SAT signal (supervisory audio tone), which supervises the connection quality between a mobile telephone and base stations. A mobile station must be capable of receiving the SAT signal, of interpreting it correctly as a predetermined nominal frequency, and of retransmitting the regenerated SAT signal to the base station. The nominal frequencies of the SAT signal are 5970, 6000 or 6030 Hz. At the time of a call set-up and in a hand-off situation, a mobile telephone receives information on the available SAT frequency from a digital control message transmitted by the base station, in which the assumed frequency is given. If the mobile telephone does not receive any SAT signal within a predetermined period, or if the received frequency differs from that given in the control message, the telephone call is disconnected. It is known in the art to use an analog phase locked loop to detect the SAT signal in a modem circuit of a subscriber apparatus. The analog phase locked loop counts the pulses of its output signal. In this prior art system the incoming SAT signal is applied via a STIF bandpass filter to an analog comparator, in which a rectangular wave is formed from the signal. Thereafter, the signal is applied to an analog phase lock loop and to an SAT frequency detector. The detector functions by comparing the frequency of the received rectangular wave with the crystal frequency of the modem circuit. The comparison takes place with the aid of a counter chain, and the modem circuit reports the result once in 85 ms. The disadvantages of a system such as this are that it has too low a tolerance for input signal noise, has phase noise in the output signal, which is manifest in a widening of the spectrum on both sides of the SAT frequency, requires a rather large number of components, requires surface area on the circuit board, and needs to be tuned during production.
It is also known in the art to use systems based on a digital phase locked loop, for SAT signal detection. The systems attempt to eliminate the problems caused by an analog phase loked loop. With these system there is used, for example, a modem circuit which is made up of three microcircuits, one of which contains the SAT components. The size of this circuit is approximately 3000 gate equivalents, and it is packed on an 80-pin IC-package. The detector part of this circuit examines only the locking of the phase locked loop, and it is rather large. Another modem circuit has two microcircuits, which require as many as seventeen external components in order to function. In spite of the digital phase locked loop its detector part is based on detecting, the locking of the phase lock loop by analog integration. Also other known systems require external components.